My advice if you're going for all the achievements is to complete additional jobs in the city and collectibles achievements first as you'll should get enough money to complete it, just go to vendor near basso's location and buy all upgrades. However if you finished game and don't have 40k gold the best method is to buy cheaper gear upgrade from vendor and buy blast arrows and sell them. Repeat the buy&sell until achievement pops up. You'll need around 10 k to start with and then the process should take about 5-10 minutes
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